# Custom string transform function
## Introduction
The **custom string** transform function is used to map a given string to a target field. This string can be static, or you can reference [flow variables](/product-documentation/process-flows/building-process-flows/dynamic-variables/flow-variables/referencing-flow-and-meta-variables-in-mapping-transformations.md) and [cached data](/product-documentation/process-flows/building-process-flows/process-flow-shapes/advanced-shapes/cache/referencing-a-cache-in-mapping-transformations.md).
{% hint style="info" %}
If you've added/updated a [map shape](/product-documentation/process-flows/building-process-flows/process-flow-shapes/standard-shapes/map-shape.md) before, you'll be used to selecting a **source** field and a **target** field. However, when a **custom string** transformation is used we don't select a **source** field - the **custom string** transformation is our data source.
{% endhint %}
## Adding a custom string transform
**Step 1**\
In your process flow, access settings for your map shape:
**Step 2**\
Select the **add transform** button for the required mapping rule - for example:
**Step 3**\
Click the **add transform** button:
**Step 4**\
Click in the **name** field to access a list of all available transform functions, then select **custom string**:
**Step 5**\
Move down to the **custom string** field and enter your required text or variables - for example:
{% hint style="info" %}
For more information about referencing flow variables in a custom string, please see our [Referencing flow variables in field mapping transformations](/product-documentation/process-flows/building-process-flows/dynamic-variables/flow-variables/referencing-flow-and-meta-variables-in-mapping-transformations.md) page. For more information about referencing cached data in a custom string, please see our [Referencing a cache in mapping transformations](/product-documentation/process-flows/building-process-flows/process-flow-shapes/advanced-shapes/cache/referencing-a-cache-in-mapping-transformations.md) page.
{% endhint %}
**Step 6**\
Accept your changes:
...then save the transformation:
**Step 7**\
Now you can select a target field in the usual way - for example:
...then:
...then:
**Step 8**\
Once your mapping is complete, the row should be displayed without a source field - for example:
From here you can save changes or add more mapping rules as needed. Next time the process flow runs, the **custom string** (or associated values from variables) will be mapped to the given target field.
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